I ran a quick thermal study in 2D simplificaiton with the mesh at a higher density and the heat power through the pipe came out as 750 kW. The 3D model you were using was coming out as 650 kW but that mesh was a bit coarser (larger elements). Have you tested for mesh independence on your FEA models by mesh element size until the results reach a converged solution (no change in results with decreased element sizes)?

EDIT: Do you have your hand calculations/equations that you can photograph and post up to look through too?